Although we used to believe that all retrotransposition occurred in the germ line in humans, over the past 10 years it has become clear that somatic retrotransposition, especially of LINE1s (L1s), is prominent, at least in the brain and in epithelial cancers. A number of groups have shown retrotransposition in neural precursor cells and the adult brain. Likewise, a number of groups including our own have shown significant retrotransposition in gastrointestinal cancers. A key finding of our studies is that most L1 insertions in GI cancers occur very early in cancer development. Indeed, some of the insertions are found in low concentration (detected only by nested PCR) in normal cells, but they become essentially clonal in the cancer (detected by conventional PCR). ln this application, we strive to answer two big questions in the field. First, does significant retrotransposition occur in adult somatic tissues outside of the brain and, if so, at what frequency? In this aim we will study L1 insertions in single normal cells near the tumors to find what fraction of the insertions in the tumor are already present in normal cells. We will determine if retrotransposition rate is greater in normal cells or in tumor cells. We will also stuy the frequency of insertions in single normal cells not associated with cancer and from various organs to determine if some tissue types have a greater propensity for retrotransposition than others. The second big question is: Does L1 somatic retrotransposition have any significant role in the etiology of cancer? We need to determine whether we can alter the cellular phenotype towards normal by deletion of somatic L1 insertions found in cancer-related genes in cancer-derived cell lines.